A difficulty regarding crankcase-scavenged engines is to provide a homogeneous air-fuel mixture to the combustion chamber. This can be achieved by so called long transfer ducts, which however tends to make the crankcase complicated and bulky. For two-stroke engines provided with additional air to the transfer ducts it is important to keep the air in the transfer ducts separated from the air-fuel mixture, in order to as far as possible prevent the air-fuel mixture from the transfer ducts to disappear out through the exhaust port. This separation, also called stratification, is promoted by making the transfer ducts long and narrow, thus preventing, or at least reducing, mixing of different scavenging gases.
The length is also adapted to the desired performance of the tool and its engine. Long transfer ducts for high torque at low speed and shorter ducts for high torque at high speed. A cylinder of the above-mentioned kind is connected to the crankcase in a parting plane essentially perpendicular towards the cylinder bore, usually with a sealing intermediate layer, such as a gasket. Either the parting plane can be located entirely above the center axis of the crankshaft bearing, a so called “short” cylinder, or the parting plane can be located essentially as high as the center axis of the crankshaft, a so called “long” cylinder.
In engines provided with additional air to the transfer ducts, as well as in conventional, high-performance engines, the transfer ducts are closed, i.e. they are separated from the cylinder bore by means of an intermediate wall. Usually closed transfer ducts are vaulted out from the cylinder body for providing the scavenging gases a desired direction into and out from the cylinder bore. This design will lead to difficulties at die-casting of the cylinder body since the direction of the transfer ducts will vary. However US 2003/0106507 A1 shows a cylinder of this type. Each transfer duct runs in a radial direction away from its transfer port and has a vaulted top part and lower part parallel with the cylinder bore. This cylinder is possible to die cast, but it has some clear disadvantages. The flow of additional air from the inlet via the piston recess and the transfer port and down into the transfer channel is slowed down by a number of sharp bends that creates a high flow resistance. From the piston recess there is first a 90° bend into the upper radial part of the transfer duct followed by a 90° bend down to the bore—parallel section of the duct. This creates a high flow resistance that reduces the amount of additional air that can be added, and therefore also the available reduction of exhaust emissions. Further these bends are also followed by a sharp bend in the parting plane.
US 2002/0043227 A1 shows a cylinder with a transfer duct that leads from the transfer port in a tangential direction. Thereafter follows a very strong bend, more than 150 degrees, to make the transfer duct meet the parting plane almost directly below the transfer port. A cutout in the lowest part of the cylinder opens each transfer duct directly in the parting plane. The tangential flow from the transfer port is an advantage compared to US 2003/0106506 A1, but the shape of the other parts of the transfer ducts results in a number of drawbacks:                The strong bend, more than 150 degrees results in a fairly high flow resistance.        The transfer duct will meet the crankcase in a very oblique angle, resulting in high flow resistance.        The transfer duct will lye on the side of the cylinder below the transfer port. This will restrict the flow of cooling air around the cylinder.        The transfer duct will not continue in the crankcase, but will open up in the parting plane. Therefore it is not possible to adapt its total length by just adapting the crankcase. The crankcase is more specifically connected to each tool application than the cylinder is.        